PrepTest 63, Section 4, Question 21

Until the 1950s, most scientists believed that the geology of the ocean floor had remained essentially unchanged for many millions of years. But this idea became insupportable as new discoveries were made. First, scientists noticed that the ocean floor exhibited odd magnetic variations. Though unexpected, this was not entirely surprising, because it was known that basalt�the volcanic rock making up much of the ocean floor�contains magnetite, a strongly magnetic mineral that was already known to locally distort compass readings on land. This distortion is due to the fact that although some basalt has so-called "normal" polarity�that is, the magnetite in it has the same polarity as the earth's present magnetic field�other basalt has reversed polarity, an alignment opposite that of the present field. This occurs because in magma (molten rock), grains of magnetite�behaving like little compass needles�align themselves with the earth's magnetic field, which has reversed at various times throughout history. When magma cools to form solid basalt, the alignment of the magnetite grains is "locked in," recording the earth's polarity at the time of cooling.

As more of the ocean floor was mapped, the magnetic variations revealed recognizable patterns, particularly in the area around the other great oceanic discovery of the 1950s: the global mid-ocean ridge, an immense submarine mountain range that winds its way around the earth much like the seams of a baseball. Alternating stripes of rock with differing polarities are laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the next with reversed polarity. Scientists theorized that mid-ocean ridges mark structurally weak zones where the ocean floor is being pulled apart along the ridge crest. New magma from deep within the earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. Over millions of years, this process, called ocean floor spreading, built the mid-ocean ridge.

This theory was supported by several lines of evidence. First, at or near the ridge crest, the rocks are very young, and they become progressively older away from the crest. Further, the youngest rocks all have normal polarity. Finally, because geophysicists had already determined the ages of continental volcanic rocks and, by measuring the magnetic orientation of these same rocks, had assigned ages to the earth's recent magnetic reversals, they were able to compare these known ages of magnetic reversals with the ocean floor's magnetic striping pattern, enabling scientists to show that, if we assume that the ocean floor moved away from the spreading center at a rate of several centimeters per year, there is a remarkable correlation between the ages of the earth's magnetic reversals and the striping pattern.

Until the 1950s, most scientists believed that the geology of the ocean floor had remained essentially unchanged for many millions of years. But this idea became insupportable as new discoveries were made. First, scientists noticed that the ocean floor exhibited odd magnetic variations. Though unexpected, this was not entirely surprising, because it was known that basalt�the volcanic rock making up much of the ocean floor�contains magnetite, a strongly magnetic mineral that was already known to locally distort compass readings on land. This distortion is due to the fact that although some basalt has so-called "normal" polarity�that is, the magnetite in it has the same polarity as the earth's present magnetic field�other basalt has reversed polarity, an alignment opposite that of the present field. This occurs because in magma (molten rock), grains of magnetite�behaving like little compass needles�align themselves with the earth's magnetic field, which has reversed at various times throughout history. When magma cools to form solid basalt, the alignment of the magnetite grains is "locked in," recording the earth's polarity at the time of cooling.

As more of the ocean floor was mapped, the magnetic variations revealed recognizable patterns, particularly in the area around the other great oceanic discovery of the 1950s: the global mid-ocean ridge, an immense submarine mountain range that winds its way around the earth much like the seams of a baseball. Alternating stripes of rock with differing polarities are laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the next with reversed polarity. Scientists theorized that mid-ocean ridges mark structurally weak zones where the ocean floor is being pulled apart along the ridge crest. New magma from deep within the earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. Over millions of years, this process, called ocean floor spreading, built the mid-ocean ridge.

This theory was supported by several lines of evidence. First, at or near the ridge crest, the rocks are very young, and they become progressively older away from the crest. Further, the youngest rocks all have normal polarity. Finally, because geophysicists had already determined the ages of continental volcanic rocks and, by measuring the magnetic orientation of these same rocks, had assigned ages to the earth's recent magnetic reversals, they were able to compare these known ages of magnetic reversals with the ocean floor's magnetic striping pattern, enabling scientists to show that, if we assume that the ocean floor moved away from the spreading center at a rate of several centimeters per year, there is a remarkable correlation between the ages of the earth's magnetic reversals and the striping pattern.